Topcoating compositions for automobiles, which generally contain a polymer having a hydroxyl group and a melamine curing agent, have been used for years. The cured films of the compositions have poor acid resistance and may often give rise to some coating defects when exposed to acid rain which has now become serious problems. It is believed that the defects are brought about from the triazine nucleus of the melamine resin and always happen as long as the malamine resin is used as curing agent.
A coating composition which does not employ the melamine resin curing agent has also be proposed. The coating composition employs a polyisocyanate curing system, but is always associated with toxic problems due to isocyanate groups.
In order to obviate the above mentioned problems, the present inventors already proposed a thermosetting resin composition which comprises a polymer having a half-esterified acid anhydride group, a hydroxyl compound and an epoxy compound (U.S. patent application Ser. No. 389,903 corresponding to EP-A 353,734). The composition has excellent coating properties, but further improvements of water resistance and weather resistance are desired.